The method for producing a white light emitting device that has been actively developed in worldwide in recent years is roughly divided into a phosphor application method, i.e., a single chip method for obtaining a white color by adding a phosphor over a blue or ultraviolet light emitting device, and a multi-chip method for obtaining a white color by combining a plurality of light emitting chips in a multi-chip con-figuration.
Concretely, in the representative method for making a white light emitting device in the aforementioned multi-chip configuration, three chips of RGB (Red, Green, Blue) are combined to manufacture a white light emitting device. However, such a method is problematic in that the output of each chip changes according to the non-uniformity of an operating voltage of each chip and an environmental temperature, thus changing a color coordinate. Due to this problem, the multi-chip method can be properly applied for the purpose of special lighting requiring display of various colors by adjusting the brightness of each light emitting device through a circuit configuration rather than by making a white light emitting device.
Under this background, as a preferable method serving as a method for making a white light emitting device, mainly used is a system in which a blue light emitting device relatively easy to manufacture and excellent in efficiency and a phosphor excited by the blue light emitting device to emit a yellow color are combined. As a representative example of such a system for emitting a white light using a phosphor, can be included a system in which Yttrium Aluminum Garnet (YAG) phosphor using a blue light emitting device as an excitation light source and using cerium ions (Ce3+) serving as trivalent rare earth ions, that is to say, a YAG:Ce phosphor, is excited by excitation light coming from the blue light emitting device.
The white light emitting device can be used in various types of packages according to the field of use. The white light emitting device is roughly divided into a micro chip light emitting diode manufactured in a surface mounting device (SMD) type for use in backlighting of cell phones and a vertical lamp type for use in electric signs, solid state display devices and image display.
Meanwhile, indexes used for analyzing the optical characteristics of a white light emitting device include a correlated color temperature (CCT) and a color rendering index (CRI).
When an object emits the same color visible light as that radiated by a black body at a certain temperature, the black body's temperature is referred to as the color temperature of the object. This temperature is defined as the correlated color temperature (CCT). The higher the color temperature, the colder and more bluish white the light will be. That is to say, if the color temperature of a white light is low, the color becomes warmer, while if the color temperature of the same white light is high, the color becomes cooler. Thus, by adjusting the color temperature, even the characteristics of special lighting requiring various color senses can be satisfied.
The white light emitting device using the YAG:Ce phosphor is problematic in that it has more or less a high color temperature ranging from 6000 to 8000 k.
The color rendering index identifies the degree of color shift objects undergo when illuminated by an artificial light source as compared to natural outdoor sunlight. If the color appearance of the object does not change when exposed to the sunlight, the CRI is 100. That is, the color rendering index is an index that measures how closely an artificial light source matches the natural colors of sunlight, and ranges from 0 to 100. Thus, the closer the CRI of a white light source is to 100, the more closely colors perceived by human eyes will appear as they do in sunlight.
At present, an incandescent lamp has a CRI of 80 or more and a fluorescent lamp has a CRI of 75 or more, while a commercialized white LED has a CRI of approximately 70 to 75, which is not very high.
Consequently, a conventional white light emitting device using a YAG:CE phosphor has a problem of a relatively high color temperature and a relatively low color rendering index. Besides, since it only employs the YAG:CE phosphor, it is difficult to control color coordinate, color temperatures and color rendering indexes.
Moreover, since the YAG:Ce is thermally degraded at a relatively high rate under a temperature of 100° C. or higher, and uses Y2O3 out of natural materials and requires a high temperature heat treatment of 1500° C. or higher for synthesizing the YAG:Ce, the YAG:Ce is disadvantageous in terms of production cost.
Further, in case of doping trivalent rare earth ions in order to shift the main emission peak of the YAG:Ce to a red region, the light emission luminosity is decreased.